1. Field of the Invention
This invention relates to improvements in an active suspension system for controlling an vehicle body attitude in response to a change in vehicle body attitude thereby to maintain the vehicle body in a flat or horizontal state, and more particularly to a device provided in the suspension system, for controlling the damping force of each of the fluid-operated cylinders forming part of the suspension system in accordance with a vehicle cruising condition.
2. Description of the Prior Art
Active suspension systems for automotive vehicles have been proposed and many are now put into practical use. One of them is disclosed in Japanese Utility Model Provisional Publication No. 1-116813, and is arranged to have a hydraulic cylinder disposed between a vehicle body and each road wheel. The pressure of a hydraulic fluid to be supplied to the hydraulic cylinder is controlled by a pressure control valve. The pressure control valve is in turn controlled by a vehicle attitude control device in a manner such to suppress an attitude change of the vehicle body. A flow restrictor is provided between the pressure control valve and the hydraulic cylinder to restrict the flow of the hydraulic fluid. Additionally, the pressure chamber of the hydraulic cylinder is connected through a vibration attenuation or damping valve to an accumulator.
The flow restrictor between the pressure control valve and the hydraulic cylinder functions to damp vibration having frequencies within a resonance frequency region of a sprung mass (vehicle body) of the vehicle, the vibration being applied from a road surface to the vehicle body. The accumulator and the attenuation valve directly connected with the hydraulic cylinder pressure chamber function to damp out and absorb vibration having frequencies within a resonance frequency region of the unsprung mass (road wheel) of the vehicle.
However, difficulties have been encountered in the above; discussed conventional active suspension system as follows. The respective attenuation constants of the above-mentioned flow restrictor and the above-mentioned attenuation valve are set at constant or fixed values. Accordingly, if the attenuation constant of the attenuation valve directly connected with the hydraulic cylinder is set relatively high, in order to sufficiently damp an vibration input at the frequencies within the unsprung mass resonance frequency region, the vibration transmission force in a high frequency range near the unsprung mass resonance frequency region is increased. This adversely affects the riding quality or occupant comfort of the vehicle. In contrast, if the flow restrictor between the pressure control valve and the hydraulic pressure is set to have a relatively high attenuation constant, in order to sufficiently damp the vibration input at the frequencies within the sprung mass resonance frequency region, the vibration transmission force on the higher frequency side than the sprung mass resonance frequency region thereby also adversely affects the riding quality or comfort of the vehicle.